Method of electrically heating an aqueous electrolyte



United States Patent O 3,469,074 METHOD OF ELECTRICALLY HEATING ANAQUEOUS ELECTROLYTE Joseph Bernard Cotton and Brenda Sanderson, SuttonColdfield, England, assignors to Imperial Metal Industries (Kynoch)Limited, London, England, a corporation of Great Britain No Drawing.Filed May 25, 1964, Ser. No. 370,115 Claims priority, application GreatBritain, May '31, 1963, 21,936/63 Int. Cl. H05b 3/60 US. Cl. 219-284 3Claims ABSTRACT OF THE DISCLOSURE There is provided an electrode andprocess of using the electrode as a heating element in boilers. Theelectrode consists of a coating of iridium, rhodium or alloys thereof,especially alloys with platinum, painted or plated on a titanium base.The electrodes are for use with an alternating current and have theadvantage that the electrode will not corrode even if the said coatingis pervious and contains pores. When water containing chlorides is usedin such a boiler, it is preferred that a soluble alkali metal phosphatebe dissolved in the water.

This invention relates to electrode boilers and in particular toelectrodes for use in and a method of operation of such boilers.

Electrode boilers are usually operated by alternating current and theelectrodes must be constructed of a material which is capable of passinga high current to the electrolyte, usually water, without corroding.Among the materials which have been proposed for use as electrodes forsuch boilers are cast iron, silicon-iron, nickel-copper alloys,copper-base alloys, aluminium and stainless steel, but it has been foundthat corrosion in various degrees occurs dependent upon thecharacteristics of the water.

According to the invention there is provided an electrode in which atleast the surfaces for passing a current into an electrolyte consist ofiridium or rhodium or of an alloy of iridium and rhodium or of an alloyof iridium or rhodium and platinum or of an alloy of iridium and rhodiumand platinum.

Conveniently, such an electrode may comprise a construction of titaniumor a titanium-base alloy coated at least on such surfaces with one ofsaid metals or alloys hereinafter referred to as iridium, rhodium ortheir specified alloys.

An alloy of 30% iridium and 70% platinum has been found to besatisfactory.

Platinum itself is effective as an electrode surface for only a shorttime because the metal oxidises and it is clearly uneconomic.

Platinum coated by either plating or painting titanium is likewiseunsatisfactory, because the platinum quickly strips off. If partiallycoated titanium anodes are used, the bare titanium corrodes,particularly in the presence of chlorides.

An electrode in accordance with the invention may be constructed bymaking the electrode shape from titanium or titanium-base alloy andcoating the surfaces from which the current is to pass to theelectrolyte by plating or painting with iridium, rhodium or theirspecified alloys. Such alloys are commercially prepared as paints.

The electrode may be in the form of a wire mesh or a sheet of iridium,rhodium or their specified alloys.

The immersed portions of the conductors to the electrodes may be madefrom titanium or titanium-base alloy provided the method of operatingthe boiler is modified as hereinafter described to prevent corrosion ofthe titanium, particularly in the presence of chlorides. The use oftitanium conductors which act also as supports for the actual electrodesclearly is advantageous and simplifies construction.

When an alternating current is being conducted between tWo titaniumelectrodes immersed in water, the protective film on the surface of thetitanium will break down and corrosion will ensue when appreciablequantities of chlorides are present and voltages are greater than 8-10volts D.C. or between -250 volts A.C. These A.C. voltages are those mostfrequently met with commercially and would normally be used forelectrode boilers at a fairly high current density.

By the modification of the method of operating an electrode boilerreferred to above, water containing chlorides can be heated or boiled atcommercial voltages without corrosion of uncoated titanium used in theconstruction of the electrodes occurring and this is achieved byproviding conditions in the boiler in which a protective film ismaintained on the titanium. Such conditions prevail when the watercontains a soluble phosphate.

A suitable soluble phosphate is an alkali metal hexametaphosphate,preferably the sodium compound commercially available under theregistered Trademark Calgon. An addition of 0.2% w/v (2 gms./litre) ofsodium hexametaphosphate has been found to be satisfactory. Where it isdesired to boil deionised water, the same amount of sodiumhexametaphosphate may be used to impart sufiicient conductivity to thewater without risk of corroding the electrodes.

Tests have been carried out on a water having the following composition:

Parts per million Conductivity 680 recip. megohms.

When such a water is boiled using A0. at 240 volts and a current of 2.0amps. corrosion of a pair of titanium electrodes occurs.

In an example of the invention an electrode boiler having A5" diametercommercially pure titanium electrodes completely coated with iridiumspaced 6" apart and immersed 3 was used. Three litres of water of theabove compositions was boiled efficiently and maintained at boilingpoint for 300 hours using an AC. 240 volt, 50 cycles/ second supply at acurent of 0.8 amp. when cold rising to 2.0 amps. at boiling point. Nocorrosion of the electrode occurred.

A similar procedure was carried out using electrodes completely coatedwith 70% platinum, 30% iridium and electrodes completely coated withrhodium without corrosion occurring. The rhodium-coated electrodes weretested for more than 2000 hours.

Other conventional electrode materials, as before mentioned, corrodeunder such conditions even in the presence of the soluble phosphate.

We claim:

1. A method of heating an aqueous electrolyte comprising at leastpartially immersing in said electrolyte at &

least two electrodes which have a base support material selected fromthe group consisting of titanium and titanium-base alloys, and a coatingon said base support material selected from the group consisting ofiridium, rhodium, and alloys thereof, and iridium, rhodium and alloysthereof further alloyed with platinum, and applying an alternatingcurrent to the electrodes, and wherein a soluble phosphate in an amountof at least about 0.2% weight/volume is added to the aqueous electrolytewhen chlorides are contained in the electrolyte.

2. A method as claimed in claim 1 in which the soluble phosphate is analkali metal hexametaphosphate.

3. A method as claimed in claim 2 in which sodium hexametaphosphate ispresent in an amount equivalent to 0.2% weight in volume.

4 References Cited UNITED STATES PATENTS 12/1926 Dantsizen 219-2842/1954 Livingstone 219-284 5/1964 Miller et al. 204-496 4/1967 Teel204-280 X FOREIGN PATENTS 1/ 1946 Great Britain. 4/ 1953 Great Britain.

ANTHONY BARTIS, Primary Examiner US. Cl. X.R.

